Re: [Libguestfs] Concurrent scanning of same disk

On Thu, May 28, 2015 at 10:57:51AM +0300, NoxDaFox wrote: > 2015-05-28 10:40 GMT+03:00 Richard W.M. Jones <rjones(a)redhat.com&gt;: > > > On Thu, May 28, 2015 at 10:33:48AM +0300, NoxDaFox wrote: > > > To create the snapshots I'm using the libvirt command snapshotCreateXML > > > with no flag set. Does libvirt support consistent snapshotting or shall I > > > rely on QEMU backup new feature only? > > > > According to: http://wiki.libvirt.org/page/Snapshots > > virDomainSnapshotCreateXML is only consistent if the guest is paused > > during the operation. > > > > The new qemu feature is called drive-backup > > (http://wiki.qemu.org/Features/IncrementalBackup). Unless things > > changed recently, it is not exposed through libvirt, so the only way > > to use it is by sending qemu monitor commands > > ( > > http://kashyapc.com/2013/03/31/multiple-ways-to-access-qemu-monitor-proto... > > ). > > > > This is all pretty bleeding edge. I still think you'd be better off > > just ignoring snapshots that fail and moving on to the next one. > > > > Rich. > > > I might be missing something then as the guest is actually paused during > the acquisition of the snapshot. > > I pause the guest, take a screenshot, a core dump and a snapshot, then I > resume the guest. Proof is that I can clearly analyse the memory core dump > without any problem. Note a core dump doesn't normally include the guest's disk. It just contains the guest's memory, so it's not relevant for consistency. > Maybe I am breaking the disk's consistency once I extract the dump through > the qemu-img command? > > The command is: > qemu-img convert -f qcow2 -o backing_file=guest_disk.qcow2 -O qcow2 -s > snapshot_n guest_disk.qcow2 new_disk_for_libguestfs.qcow2 Is the guest paused when you do this? If not, then this will create an inconsistent snapshot. > Could it be that, as the backing file is pointing to the guest's disk which > will evolve in time, when guestfs tries to read the data sees > incosistencies? qemu-img convert makes a full copy, so it's not guestfs that's the problem, but qemu-img. The copy is not done instantaneously. > The guest_disk.qcow2 is a COW clone of a base_disk.qcow2, what if I rebase > the new_disk_for_libguestfs.qcow2 to the base_disk.qcow2? Many copies and snapshots. I'm thoroughly confused ... Anyhow, unless you either make a full copy while the guest is paused, *or* you use a point-in-time snapshot feature of either qemu (drive-backup) or your host filesystem, you're not making a consistent snapshot. Rich.